This invention relates to liquid crystal based acoustic imaging systems employed in nondestructive testing, and more particularly, to the construction of the liquid crystal cells for use in such testing.
Nondestructive testing systems are used both industrially and medically in order to examine objects for internal features, defects, or the like. There are several types of nondestructive testing devices. In ultrasonic systems, the object is insonified with acoustic energy from an ultrasonic transducer and transmitted or reflected acoustic energy is analyzed for flaws or other internal features of the object.
In one specific form, the object is insonified using an ultrasonic transducer and the acoustic energy passing through or reflected from internal features of the object is detected by and is displayed on a liquid crystal cell. In such a system, the transducer, object and cell are acoustically coupled with a medium such as water.
Typical liquid crystal cells include a layer of liquid crystal material between 5 and 200 microns thick, which is usually of the nematic type exhibiting homeotropic alignment. The liquid crystal layer is disposed and encapsulated between a pair of acoustically transparent covers or substrates. A peripheral seal is provided between the covers for spacing the covers from each other and encapsulating the liquid crystal layer therebetween.
Liquid crystal cells in which a layer of liquid crystal material is encapsulated are known in the art. See, for example, U.S. Pat. Nos. 3,597,043 and 3,991,606 Dreyer; 3,707,323 Kessler; U.S. Pat. No. 3,831,434 Greguss; U.S. Pat. No. 4,338,821 Dion; and U.S. Pat. Nos. 4,379,408 and 4,506,550 Sandhu. See also, "PseudoHolographic Acoustical Imaging with a Liquid Crystal Converter", Dion et al, Acoustical Imaging, Vol. 10, Plenum Press, 1982. With regard to the cell covers, the art generally discloses the use of glass, sometimes a single sheet, as the cover material. Some of those patents disclose a specific thickness, and Greguss discloses that the thickness of the glass cover through which acoustic energy enters the cell is governed by the relationship t=n .lambda./2, where: t=thickness, n is an integer greater than or equal to 1, and .lambda. is the wavelength of the acoustic energy in the entering cover material. When n=1, about 100% transmission is achieved where the ultrasonic beam is incident on the cover at normal and near normal angles (e.g., between .+-.12.degree. of normal). Cells 2".times.2" have been successfully made and used where n=1. However, when n&gt;1 and the beam is not normal or near normal, then transmission is less than about 100% and image quality degrades due to factors such as multiple reflections, etc.
In order to permit liquid crystal cells to be used in situations where the beam is not at normal incidence, but at an angle, covers formed of laminated layers have been disclosed. See, for example, Dion U.S. Pat. No. 4,338,821 and Sandhu U.S. Pat. No. 4,379,408. Sandhu discloses laminated covers where the layer thickness is much, much less than .lambda./4 (i.e., t&lt;&lt;.lambda./4). Dion discloses stratified walls having three layers d.sub.1, d.sub.2 and d.sub.3, where d.sub.1 and d.sub.3 are between 100 and 200 microns thick and d.sub.2 is between 15 and 125 microns thick. The Dion layers are not governed by the n .lambda./2 relation.
It has been found that large size cells flex and are not sufficiently rigid. For example, a 6".times.6" single sheet glass cover where the thickness is governed by n .lambda./2, n=1 and the ultrasonic frequency is 3.5 MHz, is about 728 microns or 0.028 inches thick and may flex. Such flexing causes undesirable nonuniformities in liquid crystal layer thickness, which in turn has resulted in changes in image quality. At greater thicknesses (i.e., where n is 2, 3, 4 . . . ), the cover is more rigid, and normal alignment of the beam and cell cover becomes more critical. In other words, there is less tolerance for angular incidence.
Prior art cells, such as Dion, have laminated covers, and while providing increased angular tolerance, may exhibit undesirable flexibility or undesirable layer or ply thickness variations. Furthermore, handling of the very thin plies or layers needed for the laminated covers may pose problems in the large scale manufacture of liquid crystal cells.
It is therefore an object of this invention to provide a cell construction which will allow the ultrasound beam to be detected over a large angular range about the cell normal.
It is another object of this invention to provide a cell cover construction which is more rigid than prior constructions.
It is further object of this invention to enhance angular transmission and rigidity.
It is yet another object to provide cell covers which are readily manufactured.
These and other objects of this invention will become apparent from the following disclosure and appended claims.